Electronic manufacturers use printed substrates to connect integrated circuits (ICs) electrically, and other electrical components such as capacitors and resistors. ICs are made up of electronic components linked together by conductive connections to form one or more functional circuits. A substrate is a relatively flat and rigid structure that provides mechanical support for a die in the IC package, transmits signals to and from the IC, and can also transfer heat that is generated during the operation of the IC. The IC package may be applied to a circuit board assembly that includes systems of interconnected IC packages to form an electronic device such as a computer or a cellular phone.
Printed circuit boards (PCBs) are one type of printed substrates which typically include a number of electrical and non-electrical layers. For example, the electrical layers may include copper traces that serve as routing nets between components attached to the PCB, and layers that serve as connections to power and ground. Fiberglass or epoxy resin dielectric materials are used to separate each of the layers.
A coating referred to as solder mask is placed on the top and/or bottom of the PCB to prevent solder from flowing freely on the board, and typically has a green color. PCBs also include silkscreen layers for documentation. For example, reference designators and other text are printed on the top and/or bottom of a PCB using a process similar to that used for printing t-shirts.
One problem with carrier substrates such as PCBs is that thermal differentials on the board are not easily detectable. Thermal differential patterns on particular areas of a PCB may provide useful diagnostic information. Current methods require thermal differentials to be assessed with off-line equipment or thermal sensors placed across the surface of the PCB.